Composition for treating metal surfaces



United States Patent 3,284,319 COMPOSITION FOR TREATING METAL SURFACESUno T. Hill, Gary, Ind., assignor to Inland Steel Company, Chicago,11]., acorporation of Delaware No Drawing. Filed Jan. 8, 1965, Ser. No.424,209 8 Claims. (Cl. 252389) This application is a continuation-impartof co-pending United States application Serial No. 232,297, filedOctober 22, 1962, and now abandoned.

The present invention relates generally to the treatment of metalsurfaces to prevent corrosion and discoloration damage thereto and moreparticularly to a method of treating a galvanied or aluminized metalsurface to prevent formation of corrosion or wet storage stains on agalvanized or aluminized surface stored in an atmosphere having a highmoisture content and to a composition for so treating a galvanized oraluminized metal surface.

Many methods and compositions for treating galvanized or aluminizedsteel surfaces, and other metal surfaces to prevent corrosion or whiterust have been developed in the prior art, but these efforts have eitheradded significantly to the cost of the product or have not been entirelysatisfactory, and there is a continuing need for an improved and lessexpensive treatment for metal surfaces to prevent corrosion, such as theformation of white rust on galvanized and aluminized surfaces.

One of the most effective compounds employed for preventing corrosion orwet storage stains on galvanized or aluminized steel or other metalsurfaces is chromic acid or alkaline chromate salts. The effectivenessof these compounds is limited by the degree of absorption or retentionof the chromic acid or chromate salts on the galvanized or aluminizedsurface or on the surface of the other metals which are to be protectedfrom corrosion. It is, for example, customary in the art to treat agalvanized surface with a water absorbing compound, such as sodiumsilicate, to increase retention of the chromate ion on a metal surfaceand provide a more uniform chromate deposit, as in US. Patent No.2,987,427. The application of sodium silicate or similar silicates onthe surface of galvanized sheets, however, is objectionable in a numberof instances. For example, silicates interfere with die forming ofobjects from silicate treated galvanized sheets, and galvanized sheetstreated with silicates are not readily painted or bonderized prior topainting. Welding and soldering of the silicate treated galvanizedsheets are also hindered or completely prevented by the silicate film.

Various methods and compositions have also been devised by the prior artfor depositing a uniform film of chromic acid or chromate ion on a metalsurface to be protected against corrosion. In some instances a chromicacid compound is reduced by a compound containing one or more hydroxygroups. None of the presently available treating compositions, however,have been entirely satisfactory for preventing White rust formation on agalvanized or aluminized surface exposed for prolonged period to a highhumidity atmosphere.

It is therefore among the general objects of the present invention toprovide an improved metal coating composition and method of treatment ofa metal surface which is simple to prepare, use and apply to galvanizedand aluminized surfaces or other metal surfaces subject to corrosion,and which, for example, is usable directly without elaborate or costlypreliminary treatment or finishing operations, and which is relativelyinexpensive and reduces the overall cost of treating galvanized,aluminized or other metal surfaces.

It is also among the objects of the present invention 3,284,319 PatentedNov. 8, 1966 to provide an effective corrosion preventive compositionand method of treating galvanized, aluminized, or other metal surfacessubject to white rust formation thereon which will aid die formingoperations, provide an adequate basis for paint adhesion, and provide asurface coating which will aid in fluxing during soldering.

It is a further object of the present invention to provide an effectivemeans for applying a uniform chromate deposit on a sheet metal surfacein an economical manner and without interfering with or limiting theprincipal uses of the said sheet metal.

Other general and specific objects of the present invention will beapparent to those skilled in the art from the following detaileddescription of the invention and the claims which follow.

In accordance with the present invention it has been discovered that asolution or uniform dispersion of a trialkyl phosphate having from 3 to8 carbon atoms per alkyl group, including straight or branched chainalkyl groups, such as tri(2-ethyl hexyl) phosphate, has substantialsolubilizing action for a hexavalent chromium oxide compound, such aschromic anhydride (anhydrous chromic acid) and forms a composition whichwhen applied to a metal surface readily provides a uniform film ofchromium in conjunction with the phosphate on the metal surface andprotects the metal surfaces, particularly galvanized or aluminizedsurfaces, against corrosion and can be prepared to also impart improvedsecondary properties which are important in the forming and finishing ofsheet materials.

More particularly, it has ben discovered that of the known andcommercially available organic phosphate compounds, only the trialkylphosphates wherein the alkyl groups each have between 3 and 8 carbonatoms inclusive, such as tributyl phosphate, triamyl phosphate, trioctylphosphate, and tri(2-ethyl hexyl) phosphate, have significant solubilityfor a hexavalent chromium oxide compound, such as chromic anhydride(anhydrous chromic acid). These trialkyl phosphates also have been foundto have a marked solubilizing action on boric acid and water solublesalts thereof. Thus, chromic anhydride and boric acid have been found tobe solubilized or extractable into tributyl phosphate up to about tenpercent (10%) by weight, respectively, at moderately elevatedtemperatures F.), and at room temperature tributyl phosphate solubilizesor absorbs about eight percent (8%) by weight chromic anhydride and onlyslightly smaller amounts of boric acid (about 6%) on a weight basis.With the foregoing amounts of chromic anhydride dissolved or absorbed intributyl phosphate and which preferably also contain dissolved orabsorbed boric acid, it is possible to prepare a metal treatingcomposition which permits controlled deposition as a uniform film on ametal surface a chromium composition in effective and economical amountsand also permits imparting to the metal surface treated therewith otherdesirable properties, including paintability, solderability and1ubricity for die formation.

The very significant solubility of chromic anhydride and boric acid in atrialkyl phosphate makes economically feasible and possible the furtherdilution, dispersion and/ or emulsification thereof in a variety ofcommercially available vehicles or liquid carriers which are chemicallyinert toward the metal surface to be treated, including both organic andaqueous vehicles. For example, a treating composition comprisingtributyl phosphate which contains chromic anhydride or tributylphosphate which contains both chromic anhydride and boric acid, ifdesired, can be readily dissolved without impairing the effectiveness ofthe composition as hereinafter described in such commercially availableorganic solvents as naphtha, kerosene, diesel fuel, petroleum oil,gasoline, alcohol, carbon tetrachloride, trichloroethylene, benzene,toluene, xylene, and the like organic solvents or combinations thereof.

In certain applications of a corrosion inhibiting composition of thepresent invention, it is preferable to use the composition in an aqueousvehicle. And, it has been found that the tributyl phosphate containingchromic anhydride or chromic anhydride and boric acid can be readilydispersed in water by adding thereto an emulsifier, such as a mono-,di-, or tri-alkylolamine, such as monoethanolamine and triethanolamine,or any other of the commonly available wetting agents, detergents, andsurface acting agents including the alkyl sulfonateand aralkylsulfonate-type emulsifying agents and ethylene oxide condensates.

Whether the composition of the present invention is applied to a surfaceto be treated from an organic or an aqueous solvent vehicle, aconcentration of the tributyl phosphate-chromate solution is usedsufficient to provide a concentration of chromium on the treated surfacein an amount of at least about 0.25 mg. Cr per square foot andpreferably between about 0.75 and 1.0 mg. Cr. per square foot.Satisfactory coating on galvanized or aluminum coated strips using thecomposition of this invention have been produced having as much as 4.5mg. Cr per square foot without any evidence of yellow staining which istroublesome with other chromic acid treatments at much lower levels ofchromium. No practical or economic advantage is achieved, however, byproviding in excess of about 1.0 mg. Cr per square foot in a coating ofa galvanized steel strip with the present composition. And, for example,when the treating solution contains about 1% CrO by weight (.5 Cr) andabout 0.5% by weight tributyl phosphate, the resultant coating depositedon a galvanized steel strip following treatment therewith is found tocontain about 1.0 mg. Cr per square foot. The resultant coating alsocontains a proportionate amount of phosphate, borate and emulsifyingagent, if the latter are used in the treating composition. Thus, in thelatter embodiment the coating contains about 0.5 mg. tributyl phosphateper square foot and about 0.25 mg. boric acid per square foot.

In general, the chromic anhydride (CrO content of the treatingcomposition on a weight percent basis can vary between about 0.1% and byweight and the tributyl phosphate or other trialkyl phosphate betweenabout 0.1% and 99% by weight. And, where boric acid is also included,the formulation of the treating composition on a weight percent basiscan vary between about 0.1% and 10% by weight chromic anhydride, 0.1%and 10% by weight boric acid, and 80% and 99% by weight tributylphosphate. When the treating composition is applied as an organicsolvent solution the tributyl phosphate can vary between about 0.1% and30% by weight, the chromic anhydride between about 0.1% and 10% byweight, and the boric acid between about 0.1% and 5% by weight, with theremainder being a suitable organic solvent.

In the preferred embodiment of the present invention wherein thetreating composition is applied as an aqueous solution or emulsion theingredients thereof can be varied as follows:

Whereas the direct combination of anhydrous chromic acid (chromicanhydride) and tributyl phosphate at room temperature results in acombustible mixture, the presence of even a relatively small amount ofwater with the chromic anhydride enables the chromic acid solution todissolve smoothly and completely in the tributyl phosphate withoutoxidation to form a homogeneous stable chelate or complex. It isadvisable to employ a chromic acid solution having a minimumconcentration of 50% by weight water to prevent an exothermic oxidationreaction, and preferably a somewhat more diluted chromic acid solution(e.g. 20% by weight CrO is used when combining the chromic anhydridewith the tributyl phosphate to form the homogeneous active complex. Itis also possible for the tributyl phosphate which has solubilized oroomplexed therewith an aqueous chromic anhydride solution to solubilizeor absorb therewith an ap preciable amount of another metal surfaceprotective or treating agent, such as boric acid or the like. After thehomogeneous composition is formed (with or without another treatingagent solubilized or absorbed thereby), the homogeneous composition canbe diluted with an organic solvent in all proportions or dispersed inwater with any conventional emulsifying agent, such as an aralkylsulfonate, or a lower 'alkylolamine, or by mechanical emulsifyingprocedures.

In the various preparations used for treating a metal surface inaccordance with the present invention, the treating composition ispreferably maintained at a pH above about 2.5 and below about 6, andmost advantageously at about pH 5, so that none of the hexavalentchromium is reduced to trivalent chromium even after prolonged standingand use.

The treating compositions of the present invention, while preferably butnot necessarily being maintained at a temperature between about F. andF., can be readily applied to a metal surface including galvanizedsheets, strips or other articles by dipping, spraying, wiping, rolling,painting or other means of coating, And, while it is generally mostconveniently applied to a surface such as a moving galvanized strip bypassing the strip through a solvent solution or emulsion of the hereindescribed treating composition or by spraying said solution of emulsionon the galvanized strip, it is also possible to spray the tributylphosphate containing chromic anhydride and (if desired) boric acidwithout solvent dilution by means of Grammer sprays.

After application of the treating composition to the surface of anarticle, such as a continuous galvanized steel strip, by any of theavailable means, it is generally preferable to contact the metal surfacewith a wiper or squeegee or the like to insure an even distribution ofthe solution on the surface of the strip and to remove any excess of thecomposition. Particularly good results have been obtained by employingwiper rolls fabricated from polyurethane polymer. A greater coetficientof friction between the galvanized strip and the roll is realized withpolyurethane wiper rolls than between wiper rolls fabri cated fromrubber or neoprene polymer; thus overcoming a tendency of the wiperrolls to slip over the surface Without turning. And, by coupling a setof polyurethane rolls through a belt or a chain drive to a second set ofneoprene or rubber rolls a very uniform film of the treating solution isdeposited upon the strip. This coupling of the rolls has also greatlyreduced the need for frequent wiper roll changes. After wiping thesheet, the moisture remaining can be removed rapidly by blowing warm airover the surface or by simply allowing the coating to air dry duringpassage of the strip to a subsequent station.

The following specific examples are given to particularly illustrate thepresent invention, without, however, limiting the invention to theprecise ingredients and proportions used.

Example 1 An aqueous tributyl phosphate, chromic acid and boric acidsurface treating composition was prepared as herein described and havingthe following composition:

Percent by weight Twenty gallons of an aqueous chromic acid solutionformed by carefully dissolving 36.36 pounds of chromic anhydride thereinare mixed with 5 galof tributyl phosphate. The chromic compound isextracted into the organic phase by the tributyl phosphate in thepresence of the water without oxidation of the tributyl phosphate andreduction of the chromic compound. To the foregoing is added 10 gallonsof an aqueous solution containing 13.64 pounds of boric acid and 2.73gallons of monoethanolamine. Water is then added to bring the volume ofthe composition to about 50 gallons. The foregoing ingredients arestirred and complete solubilizing or emulsification is readily effected,whereupon the composition is transferred to a treating tank containing400 gallons of water and equipped with a recirculating pump.

The foregoing treating composition is maintained at a temperature ofbetween about 105 and 125 F. and is sprayed on both surfaces of acontinuously moving strip of galvanized steel. The strip is passedthrough a pair of oppositely disposed rubber wiper rolls of about 40durometer hardness to evenly distribute the treating composition and toremove any excess before the strip leaves the treating tank. Thepressure of the wiper rolls is maintained so as to provide the desiredweight of chromium in the residual coating on the treated surface (i.e..75 to 1.0 mg. Crper square foot).

The pH of the treating composition is maintained between the limits of 3and 6, and preferably as close as possible to pH 5. This can be doneconveniently by testing the preparation periodically with suitable testpaper, such as pHydrion paper. When the pHydrion paper turns a greenishcolor, the pH is too highly alkaline, and one quart at a time of aqueouschromic acid solution is added until the pH reaches the desired level. Aperiod of about 15 minutes should elapse between additions of thechromic acid solution. If the pH of the solution is too low, a quart ofaqueous monoethanolamine is added in the above manner until the pH againis about pH 5.

At the end of a large scale test run extending over a six day period,the average amount of metallic chromium on the top and bottom surfacesof a galvanized steel strip treated with the foregoing aqueouscomposition was .92 and 1.27 mg. Cr per square foot, respectively. ThepH of the treating composition maintained at an average pH of 5.1 andthe chromic anhydride content was maintained at an average weight percent of 0.99% over the entire run.

The foregoing composition and method of treating when applied to analuminum coated steel strip imparts very good corrosion resistance whichis superior to presently used treatments.

Example 2 A slightly modified treating solution was made up as follows:Boric acid (H BO in an amount of about 17.5 pounds was dissolved inabout gallons of water, and to the foregoing was added 6 gallons oftributyl phosphate containing 10% chromic anhydride (C10 and 2.5 gallonsof monethanolamine (NH C H OH). When adding chromic anhydride totributyl phosphate, at least 50% by weight of water must first be addedto the chromic anhydride to enable the extraction of the hexavalentchromium solution into the organic phase and to prevent oxidation of thecompound.

Slight stirring of the treating solution will elfect complete solubilityor emulsification of all of the chemicals which are then transferred tothe treating tank containing 400 gallons of water. Thirty-three pounds(33 lbs.) of chromic anhydride are then slowly added to the treatingtank and recirculated by means of the recirculating pump.

Following spraying of the solution onto both surfaces of a galvanizedsteel strip, even distribution and accurate control of the chemicalcoating was obtained by wiping the strip with a single set of No. 40durometer polyurethane wiper rolls..

The foregoing chemical treatment provided a residual coating on thegalvanized strip which has between 0.8 to 1.0 mg. Cr per square foot.The galvanized steel strips treated in the foregoing manner have beenproven by field tests to be equal or superior in resistance to corrosionand surface marking to the more costly two-stage chromesilicatetreatment.

Example 3 In. the same manner described in Example 2, the followingweight percentage composition was prepared:

Percent Chromated tributyl phosphate 2 Monoethanolamine 3 Chromicanhydride l Boric acid 1 Remainder water.

The resulting composition was found to impart good corrosion resistancewhen applied to a galvanized steel surface.

Example 4 In the manner described in Example 1, the followingingredients in the specified amounts were prepared:

Triamyl phosphate gals 5 Chromic anhydride lbs 36.36 Boric acid lbs13.64 Monoethanolamine gals 2.73 Water, diluted to "gals" 450 The zincplate treated with the above composition in the standard 40-hour stacktest was free of white rust.

Example 5 In the manner described in Example 1, the folowing ingredientsin the specified amounts were prepared:

Trioctyl phosphate gals 5 Chromic anhydride lbs 36.36 Boric acid lbs13.64 Monoethanolamine gals 2.73 Water, diluted to gals 450 The zincplate treated with the above composition in the standard 40-hour stacktest was free of White rust.

Example 6 In the manner described in Example 1, the followingingredients in the specified amounts were prepared:

Tri(2-ethyl hexyl) phosphate 'ml 10 CF03 g H3BO3 g. Monoeth-anolamine gl5.55 Water, diluted to 1 liter with water pH 5.0.

The zinc plate treated with the above composition in the standard40-hour stack test was free of white rust.

Example 7 In the manner described in Example 1, the followingingredients in the specified amounts were prepared:

Tri(2-ethyl hexyl) phosphate ml 10 CrO g 10 HNO -sp. gr. 1.42 m1 5 H to1 liter.

The zinc plate treated with the above composition in the standard40-hour stack test was free of white rust.

Example 8 In the manner described in Example 1, the followingingredients in the specified amounts were prepared:

Tri(2-ethyl hexyl) phosphate ml 10 CrO g 10 NH O ml 10 Kyro E.O. g .1 pH4.5.

H 0 to 1 liter.

The zinc plate treated with the above composition in the standard40-hour stack test was free of white rust.

Example 9 In the manner described in Example 1, the followingingredients in the specified amounts were prepared:

Tri(2-ethyll1exyl) phosphate ml 10 CrO g 10 NPnOI-I ml 10 Kyro E.O. g .1pH 5.0.

H 0 to 2 liters.

The zinc plate treated with the above composition in the standard40-hour stack test was free of white rust.

In the preceding examples the source of chromic ion in the compositionhas in each instance consisted of chromic anhydride. It should beunderstood, however, that a water soluble chromate salt, such as thealkaline chromates including sodium chromate, ammonium chromate, or thelike can be dissolved in water and on mixing the chromate solution withthe tributyl phosphate or the equivalent triloweralkyl phosphate, thetributyl phosphate will take up the chromate ion to form a homogeneousactive complex. Similarly, in place of the boric acid, other boratesalts can be used, such as sodium borate, potassium borate, and thelike, which yields a boric acid radical in aqueous medium.

It has been found that when an aluminum coated steel strip is treated asdescribed in the foregoing Examples 1 through 9, the same improvedcorrosion resistance is imparted to the aluminum surface.

While the foregoing specific examples have relied primarily on analkylolamine as the emulsifying agent, any of the other commonlyavailable emulsifiers and detergents and the like which are genericallydesignated as surfactants can be used in the present invention. Forexample, among the emulsifying agents which can be used in the presentinvention are the aralkyl sulfonates, such as dodecyl benzene andtridecyl benzene sodium sulfonates and the aryl sulfonates, such asxylene sodium sulfonate and toluene sodium sulfonate, and lauryldiethanolamide, and lauryl alcohol sulfates. Also usable are theethylene oxide adducts to fatty and rosin acids, alkyl phenol and itsderivatives, the sulfated acrylic nitrogen products and sulfated esters,such as sulfated glycerol monostearate, aromatic sulfonate-oxidecondensates, dioctyl ester of sodium sulfosuccinate acid, propylenemonostearate, mono and di-g-lycerides of fatty acids, sulfated alcoholderivatives, polyoxyalkylene esters and sulfonates. It will be evidentthat there are a great many ionic and non-ionic emulsifiers which can beused in the present invent-ion, and none of the commonly availableemulsifiers have interfered with the satisfactory operation of the thepresent invention when used as described herein.

In the foregoing specification and in the claims appended hereto theterm metal surface is used to designate the metal surfaces which arereadily subject to attack by corrosion, particularly to the formation ofa surface discoloration and to the formation of a white surface filmcommonly designated white rust, and includes zinc, aluminum,zinc-aluminum alloy, and magnesium surfaces.

Others may practice the invention in any of the numerous ways which aresuggested to one skilled in the art by this disclosure, and all suchpractice of invention are considered to be a part thereof which fallwithin the scope of the appended claims.

I claim:

1. A composition for treating a metal surface susceptible to damage bycorrosion selected from the group consisting of zinc, magnesium andaluminum and alloys thereof comprising as an essential active ingredientan aqueous dispersion of between about 0.1% and about 30% by weighttributyl phosphate in combination with 0.1% and about 10% by weightchromic anhydride and with at least a portion of said chromic anhydridebeing dissolved by said tributyl phosphate, and said tributyl phosphateand chromic anhydride being dispersed in Water containing between about0.1% and about 30% by weight monoethanolamine as an emulsifying agent,and said aqueous dispersion having a pH of between about pH 2.5 and 6.

2. A composition as in claim 1, wherein said aqueous dispersion containson a weight basis about 1% tributyl phosphate, about 1% chromicanhydride, and about 0.6% monoethanolamine.

3. A composition as in claim 1, wherein said aqueous dispersion containson a weight basis between about 0.1% and 10% by weight boric acid.

4. A composition as in claim 3, wherein said aqueous dispersion containson a weight basis about 1% tributyl phosphate, about 1% chromicanhydride, about 0.4% boric acid, about 0.6% monoethanolamine, and thebalance of said composition being essentially water.

5. A composition for treating a metal surface susceptible to damage bycorrosion selected from the group consisting of zinc, magnesium andaluminum and alloys thereof comprising as an essential active ingredientan aqueous dispersion of between about 0.1% and about 30% by weighttri(2ethyl hexyl) phosphate in combination with between about 0.1% andabout 10% by weight chromic anhydride and with at least a portion ofsaid chromic anhydride being dissolved by said tri(2-ethyl hexyl)phosphate, said phosphate and chromic anhydride being dispersed in watercontaining between about 0.1% and about 30% by Weight monoethanolamineas an emulsifying agent, and said aqueous dispersion having a pH ofbetween about pH 2.5 and 6.

6. A method of continuous in-line treatment of a surface of an endlessstrip of metal having a surface selected from the group consisting ofzinc, magnesium and aluminum and alloys thereof which comprises;contacting a surface of said strip moving continuously past a treatingzone with a fluid aqueous dispersion having as an essential activeingredient a composition comprising between about 0.1% and about 30 byweight tri(2-ethyl hexyl) phosphate and between about 0.1% and about 10%by weight chromic anhydride with at least a portion of said chromicanhydride being solubilized by said tri(2-ethyl hexyl) phosphate to forma solution containing chromate dissolved in said tri(2-ethy1 hexyl)phosphate and said aqueous dispersion containing between about 0.1% andabout 30% by weight of an emulsifying agent selected from the groupconsisting of monoethanolamine, diethanolamine and triethanolamine withthe balance of said aqueous dispersion being water with said aqueousdispersion having a pH between about pH 2.5 and 6, continuously formingon the surface of said metal strip as said strip moves continuously pastsaid treating zone a uniform wet film containing said solution ofchromate and tri(2-ethyl hexyl) phosphorous, and continuously dryingsaid wet film to form a residual dry film containing chromium in anamount between about 0.25 and about 1.0 mg. per square foot of saidmetal surface.

7. A method of continuous in-line treatment of a surface of an endlessstrip of metal having a surface selected from the group consisting ofzinc, magnesium and aluminum and alloys thereof which comprises;contacting a surface of said strip moving continuously past a treatingzone with a fluid aqueous dispersion having as an essential activeingredient a composition comprising between about 0.1% and about 30% byweight tributyl phosphate and between about 0.1% and about 10% by weightchromic anhydride with at least a portion of said chromic anhydridebeing solubilized by said tributyl phosphate to form a solutioncontaining chromate dissolved in said tributyl phosphate and saidaqueous dispersion containing between about 0.1% and about 30% by weightof an emulsifying agent selected from the group consisting ofmonoethanolamine, diethanolamine and triethanolamine and with saidaqueous dispersion having a pH between about pH 2.5 and 6, continuouslyforming on the surface of said metal strip as the said strip moves pastsaid treating zone a uniform wet film of said solution of chromate andtributyl phosphate, and continuously drying said wet film to form aresidual dry film containing chromium in an amount between about 0.25and about 1.0 mg. per square foot of said metal surface.

8. A method as in claim 7, wherein said dispersion contains on a weightbasis about 1% tributyl phosphate, about 1% chromic anhydride, and about0.6% monoethanolamine.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCESRose et a1., Condensed Chemical Dictionary (1961), page 1159 relied on.

LEON D. ROSDOL, Primary Examiner.

JULIUS GREENWALD, Examiner.

M. WEINBLATT, Assistant Examiner.

1. A COMPOSITION FOR TREATING A METAL SURFACE SUSCEPTIBLE TO DAMAGE BYCORROSION SELECTED FROM THE GROUP CONSISTING OF ZINC, MAGNESIUM ANDALUMINUM AND ALLOYS THEREOF COMPRISING AS AN ESSENTIAL ACTIVE INGREDIENTAN AQUEOUS DISPERSION OF BETWEEN ABOUT 0.1% AND ABOUT 30% BY WEIGHTTRIBUTYL PHOSPHATE IN COMBINATION WITH 0.1% AND ABOUT 10% BY WEIGHTCHROMIC ANHYDRIDE AND WITH AT LEAST A PORTION OF SAID CHROMIC ANHYDRIDEBEING DISSOLVED BY SAID TRIBUTYL PHOSPHATE, AND SAID TRIBUTYL PHOSPHATEAND CHRONIC ANHYDRIDE BEING DISPERSED IN WATER CONTAINING BETWEEN ABOUT0.1% AND ABOUT 30% BY WEIGHT MONOETHANOLAMINE AS AN EMULSIFYING AGENT,AND SAID AQUEOUS DISPERSION HAVING A PH OF BETWEEN ABOUT PH 2.5 AND 6.